Coilovers : different size front/rear
 

Hi,
Here in Sweden, we have SFRO, which is the equivalent of your IVA in UK. They publish a manual, and you have to respect their regulations if you build a car.
One of them is that the shock absorbers (let's write "s.a." to be shorter) have to be set at a maximum angle of 20° to 30° (I guess they mean from the vertical axis).

The roadster is designed with s.a. at a 45° angle, so here in Sweden, we need to modify the position of the s.a. brackets to get a max. angle of 30°. As the s.a. will more "vertical", it will also have to be shorter (if we want to keep the wishbones and uprights as they are, and modify the design as less as possible).
Do you follow me until here ?

For more info, I publish the study I made of the different options (but certainly not the only ones) on my blog with a lot of illustrations. But let me warn you : it is quite long and can be extremely boooooring...

http://vouchtroadster.blogspot.com/2...s-to-meet.html

Anyway, after calculations, 3D simulations and so on, it appears that in order to make the smallest modifications and do it the easiest way, the front coilovers will have to be 12'' inches long, and the rear ones will have to be 11'' long.

My question is : do you see any problem having different lengths of s.a. at the front and at the rear ?

Thank you for your comments and opinions about that.

Short answer, for IVA different length no problem, for your version no idea!!

Thank you for your comment.
Why wouldn't it be ideal good for my version ? Any technical reasons ?

Thank you Phil, very interesting. I haven thought about the risk to bend the lower front wishbone if I move the lower bracket inward.
But anyway, I can't use 13'' coilovers if I move the front and rear top bracket as you tell me : they will be too long. Do you think I should recalculate to see if I can use the same length at the font and at the rear (which I think will be 11'')?

Thank you very much Phil, that is what I'm gonna do.
I also had a look at the Robin Hood, and the solution on the "zero" is quite seductive, as it also combine the head light support. I will try to work in this direction.
Thank you.
Sylvain

i would move the tops out on the fronts and use a shorter shock and spring
at the rear if you move the top out you may create a clearance issue with the rear wheel so you could modify the top suspension arm to take a bottom shock mount but i would be up rating it to take more load
hope that helps

also remember that the spring rate change with respect to the sin of the angle

example:

front spring rate is 350lbs/in IIRC @45 deg
if the spring were vertical it would need to be (350xsin45 = 247.5lbs/in)

to find the new spring rate we can take the vertical rate and apply a new angle of 60deg (measured from horizontal, dont use 30 you have to measure up from horizontal not down from vertical)

247.5 / sin60 = 285.7lbs/in

this is inly the case if the outboard pivot point stays in the same place and you move the inboard pivot point.

if you move the outboard pivot closer to the chassis then the leverage effect it has on the shock with be much greater and thus require a heaver spring rate and will also make it more lightly that you will bend or worse snap a wishbone.

example if the outboard pivot for the shock is 400mm away from the chassis pivot for the wishbone it is connected to and it supports 100KG if you half this distance you double the load. so now at 200mm it has a 200KG load and so on... this is very bad practice

making the shock more vertical is a very good thing and can be done by moving the inboard end of the shock. the outboard end of the sock should be as far outboard as possible so as to reduce the bending moment on the wishbone. wishbones are not designed to take bending loads, they are designed to take load in tension and compression.


hope this is of some help

tilly

no problem.

tilly

Thank you all for your comments.
Baz-r, I like your idea of modifying the rear upper wishbone to get a lower attach point for the shock absorber. But isn't there the risk here too to bend the camber adjuster ? (As Phil said at the beginning of these thread that there is a risk of bending the front wishbone of I increase the move the front lower bracket inward). What do you guys think ?

Phil, as I'm quite sure I will order the wishbones and rear upright from you when time (and money) will come, do you feel like doing this kind of modif ? Any ideas for the design ?

The other thing I'd like to know, is some measurements I can't have cause I don't have the parts yet :
- oustide overall diameter of a coilover with the spring. My choice will go on a 2'' body diameter. If one of you guys have the same kind of coilovers, could you please take the measurements for me : I can't find anything on Internet.
- I also need to know the distance from inside rim to inside rim of the rear wheels, with Ford 15'' wheels (ET 35-38mm), so I can check the clearance if I just move the rear upper bracket outward.

Tilly, thank you for your calculations for the spring, I will take this in consideration when I order the coilovers and springs, very useful.

Thank you in advance for you help.
Sylvain

as i pointed out it would need uprating (strengthaning) or another idea woud be to modify the plates where the shock meets the hub so the shock mount bolt is ferther inwards abit like a 7 shape extension to the rear hub just watch clearance with the top wishbone

Thank you Baz-r. Yes, modifying the rear upright is another solution I thought about too. But all of this is quite complicated. Of only I could be sure about the clearance between the shock absorber and the wheel rim, I could either keep or reject the simple solution consisting of moving the upper bracket outward. But for this, I need the measurements I described in my previous post.
So, if somebody read that, thanks in advance !
Sylvain

voucht,
have you contacted Janne E? He got his car recently registered in Sweden. Here's some pictures of his car http://www.haynes.co.uk/forums/showp...2&postcount=62. Looks to me that his front shocks are per book design.

Hi Madis,
I watched the pictures of his car last week (and I think it is beautiful) but no, I didn't know he was in Sweden. And you are right, the suspension seems to be according to the book. I send him a personal email right now.
Thank you for the tip !

In regards to the front suspension what if you were to go for inboard suspension as they are more vertical and would solve the issue.

Hi Ozzy,
Thank you for your comment.
Yes, might be an idea, but in that case, I have to check if the angle of the push-rods will not be THE issue. I also have to check if I have enough clearance between the radiator fan and the engine.

you could do a rocking upper arm design, like this

http://www.google.co.uk/imgres?q=roc...1t:429,r:7,s:0

tilly

As long as there is sufficient travel in the shorter damper you should be fine.

Tilly, yes, it would be great, thank you for the link, but honestly, I want to keep the modifications as simple as possible, so I think I will go for an extension "box" as on the Robin Hood, as Phil advised (I'll post the design of the extension as soon as it is finished, to have your opinions about it)
According to my 3D simulations, I should be able to use 11'' coilovers front and rear. Which leads to an answer for MarkB : the stroke of a 11'' is 3'', do you think it is enough ? The 13'' have a 4'' stroke, so I'm loosing about 25mm of travel.
Thanks for your comments.

In an ideal world you want to be setting the shock mounting positions 60/40. Be very careful when remounting the upper brackets as they need to be well braced and all forces directed into a braced chassis.

Thank you Mark. Sorry, but I don't know what you mean by "60/40", sounds like 60% - 40% but about what ? Can you be more specific please ?

I actually will not remounting the upper brackets higher, but they will more out, with a special assembly (a bit like on the Robin Hoods). Though, it still need to be well braced. I reopened a new thread about the design of this assembly :
http://www.haynes.co.uk/forums/showthread.php?t=7311
so your comments as well as all others are are welcome ! :)

Thanks for the help.
Sylvain